The changes in synaptic strength that underlie behavioral phenomena such as learning, memory and addiction, are usually attributed to postsynaptic changes in receptor sensitivity, but they may also reflect presynaptic regulation of the amount of transmitter released. The amount of glutamate per vesicle available for exocytotic release depends on its transport from the cytoplasm into vesicles by a family of vesicular glutamate transporter proteins, VGLUTs. VGLUT1 and 2, which account for the exocytotic release of glutamate by essentially all well-established excitatory neurons, exhibit a mutually exclusive pattern of expression in adult brain that correlates with the probability of transmitter release and the potential for plasticity. VGLUT1 also contains several protein-protein interaction domains not found in VGLUT2. The long-term goal of this K08 Mentored Clinical Scientist Development Award proposal is to understand how differences in the trafficking of VGLUT isoforms contribute to the differences in glutamate release observed at different synapses. The strategy is to study the role of C-terminal sequences in activity and trafficking of the different VGLUT isoforms. We have found that one of the polyproline motifs in the C-terminus of VGLUT1 interacts with proteins involved in synaptic vesicle recycling. We will characterize these interactions and determine whether they regulate glutamate transport by influencing vesicle filling, trafficking, or other aspects of the vesicle cycle. In addition, we will characterize and identify protein interactions with two other internalization motifs in VGLUT1. Together, these motifs may contribute to differences in the speed of vesicle filling, exocytosis and endocytosis in glutamate signaling. Increasing evidence implicates glutamate hypofunction in the pathophysiology of schizophrenia, which may involve changes in glutamate release. Thus, an understanding of the regulation of glutamate storage and release may be important in the targeting of therapeutic agents for schizophrenia